1. Field of the Invention
The present invention relates to a power amplifier for mobile communication such as mobile telephones.
2. Background Art
Recently, GaAs-HBT (Hetero-junction Bipolar Transistor) power amplifiers are widely used as power amplifiers for mobile telephones including CDMA (Code Division Multiple Access), (for example, refer to Japanese Patent Application Laid-Open No. 2004-343244). In urban areas where base stations are relatively concentrated, the power amplifiers are mainly operated at middle-and-low output. Therefore, the improvement of operation efficiency is effective for the elongation of talk time of mobile telephones in the time of middle-and-low output operation. Consequently, the improvement of operation efficiency in the time of middle-and low output operation (approx. 0 to 17 dBm) is becoming important in addition to the time of high-output operation (approx. 28 dBm). For improving the operation efficiency in the time of middle-and low output operation, a method for lowering the collector voltage of amplifying elements depending on the output power by DC/DC converters has been known.
FIG. 16 is a graph showing the relation between the output power and the distortion of an amplifying element. FIG. 17 is a graph showing the relation between the output power and the operation efficiency of the amplifying element. When the collector voltage is lowered (Vc_High to Vc_Mid) the distortion characteristics are degraded and the efficiency are improved. In addition, although distortion characteristics are improved by lowering the output power, the operation efficiency is lowered. Therefore, by lowering the collector voltage and inhibiting the output power to the range where the output power satisfies the standards of distortion characteristics, the operation efficiency is improved.
FIGS. 18 and 19 are graphs showing the temperature characteristics of the distortion of an amplifying element. The output power is 17 dBm. Since the collector voltage cannot be much lowered when the variation of distortion is large at the time of Vc_Mid as shown in FIG. 18, the operation efficiency cannot be sufficiently improved. Therefore, distortion characteristics of favorable flatness at the time of Vc_Mid as shown in FIG. 19 are desired.
In addition, a bias circuit is used in the power amplifier for supplying bias to the amplifying element. FIGS. 20 and 21 are diagrams showing conventional bias circuits. Trb1 to Trb5 denote HBTs, Rb1 to Rb5 denote resistors, Vref denotes the reference voltage terminal of the bias circuit, Vcb denotes the collector power terminal of the bias circuit, and Vbo denotes the output terminal of the bias circuit.